Sum Frequency Generation Spectroscopy of Imidazolium-based Ionic Liquids with Cyano-functionalized Anions at Gas-Liquid and Solid-Liquid Interfaces

As essential media currently used in several technological applications, the accurate molecular level description of ionic liquids (ILs) at the gas-liquid interface is of utmost importance. The inherently surface-sensitive technique, sum frequency generation (SFG) spectroscopy, in combination with bulk-sensitive vibrational spectroscopic techniques such as FTIR and Raman, have been used in this report to characterize the surface of cyano-containing ILs, such as [BMIM][SCN], [BMIM][DCA], [BMIM][TCM], and [EMIM][TCB] at the gas-liquid interface. By structural variation of the anion while keeping the cation constant, emphasis on the molecular arrangement of the anion at the gas-liquid interface was reported.

Vibrational modes seen in the C-H stretching region revealed the presence of the cation with an orientation independent of the type of anion. A similar arrangement at the surface as reported in previous studies was found. All three anions of varying symmetry, namely, [DCA]- (C2v), [TCM]-(D3h), and [TCB]- (Td) in ILs [BMIM]DCA], [BMIM][TCM] and [EMIM][TCB] were significantly tilted from the surface plane, while the linear [SCN]- in [BMIM][SCN] exhibited poor ordering, seen in the absence of its C-N stretching mode in the SFG vibrational spectra.

Likewise, all four ILs in contact with two different solid salt surfaces, BaF2(111) single crystal and solid NaCl{100}, were discussed in this report. These studies described the nature of an ionic liquid-(solid) salt interface, contributing a new understanding to the molecular-level interactions involved in salts. Results showed that [BMIM]+ cations adhered closely via Coulombic interactions to the negatively-charged NaCl{100} surface, while [SCN]-, [TCM]-, and [DCA]- anions revealed a strong electrostatic affinity to the positively-charged BaF2(111) surface. Ions of the ionic liquid adsorbed to the solid salt surface to form a Helmholtz-like electric double layer.

At the BaF2(111)-[EMIM][TCB] interface, however, a strongly-bound layer of anions populating the first layer resulted to a much larger counter-ion charge delivered near the crystal salt surface than required to effectively neutralize the initial surface charge from the crystal. As a result, strong resonances from the cation were observed at the BaF2(111) surface, suggesting a more complicated structure of the double layer at the interface, than a simple Helmholtz-type model.